A carbon-containing refractory containing a carbon material, such as graphite, pitch, carbon black and phenol resin, is excellent in thermal spalling resistance and slag resistance, and therefore is widely used in iron and steel, non-ferrous metal, cement, incinerators, waste-melting furnaces or the like. The wide variety of applications of such a refractory include inner wall bricks for converters, ladles, torpedo cars, vacuum degassing vessels or the like, monolithic refractories, continuous-casting nozzles such as submerged nozzles, and a repairing material such as spraying refractories and sintering.
However, the required quality level of steel products has been getting higher than a few years ago, as well as the required composition control level thereof has been getting stricter. This has shed light on the issue of liquid steel contamination, which is dissolution of carbon existing in a carbon-containing refractory in liquid steel (hereinafter, referred to as “carbon pickup”). In particular, magnesia-carbon bricks for converters, vacuum degassing vessels, ladles or the like originally contain graphite at content ratios of as high as 10 to 30 wt %, and users thereof are eager for reduced content ratios of graphite. To prevent the occurrences of the carbon pickup, reduction in the carbon content in a carbon-containing refractory is necessary.
Furthermore, thermal loss such as decreased temperature of liquid steel because of a high thermal conductivity of carbon, deformation of the surfaces of iron vessels, carbon monoxide emission associated with carbon combustion and other issues also necessitate the reduction in the carbon content in a carbon-containing refractory.
Reduction in the carbon content in a refractory involves the problem of lowered thermal spalling resistance of the refractory. The thermal spalling fracture resistance parameter R, which is an index of the thermal spalling resistance, is expressed as R=S(1−σ)/Eα, where S represents modulus of rupture, E represents Young's modulus (longitudinal elastic modulus), σ represents Poisson's ratio and α represents linear expansion coefficient. Reducing the carbon content in a refractory results in particular increase in the linear expansion coefficient. Furthermore, it has been reported that a low carbon content would raise the frequency of contacts between aggregate particles formed of a refractory raw material such as magnesia and accordingly the aggregates exposed to high temperatures for a long time are excessively sintered, increasing the Young's modulus E of the refractory (see Non-patent Document 1). Therefore, the fracture resistance parameter R is generally reduced as the carbon content is reduced.
Considering the above situation, researchers have been utilizing a technique wherein pitch is added into a refractory, to suppress the increases in the linear expansion coefficient and the Young's modulus for reducing the carbon content in the resulting carbon-containing refractory (for example, see Patent Documents 1 to 3). Particles of heated pitch penetrate voids existing in the internal structure of bricks or between aggregate particles to fill such voids. This inhibits the contacts between and sintering of the aggregate particles. Furthermore, this matrix portion absorbs and buffers the expansion of the aggregate particles that occurs under high temperatures, thereby suppressing the linear expansion. This seems to result in an improved thermal spalling resistance (see Non-patent Document 2). In addition, pitch densities the internal structure of bricks, and thus has the effect of improving the strength of bricks by preventing the penetration of slag, hot metal and liquid steel.
Patent Document 1 discloses low-carbon MgO—C bricks containing low-softening-point pitch that has a softening point equal to or lower than 250° C. The document states that, since the softening point of pitch being 250° C. or lower, particles of the added pitch are molten and carbonized when heated, while penetrating small voids existing in the internal structure of bricks to form carbon bonds, and as a result, the hot strength and abrasion resistance at high temperatures of the bricks are improved.
[Patent Document 1]    Japanese Unexamined Patent Application Publication No. H9-309762
[Patent Document 2]    Japanese Unexamined Patent Application Publication No. H9-132461
[Patent Document 3]    Japanese Unexamined Patent Application Publication No. H6-321626
[Non-patent Document 1]    Atsushi Torigoe, Kazuhiro Inoue and Yasuhiro Hoshiyama, “Improvement of Spalling Resistance of Low-carbon MGO—C Bricks,” Refractories, Vol. 56 [6], pp. 278-281, 2004.
[Non-patent Document 2]    Toshihiro Suruga, Eiichiro Hatae, Toshiyuki Hokii and Keisuke Asano, “Spalling Resistance and Hot Behavior of Magnesia-Carbon Bricks,” Refractories, Vol. 56 [10], pp. 498-502, 2004.
[Non-patent Document 3]    Katsufumi Shirono, Eizo Maeda, Kazuyoshi Nakai and Toshihiro Yoshida, “Effect of Pitch on Carbon Bond Formation in Castables,” Refractories, Vol. 55 [11], pp. 530-531, 2003.